feat(harness): InferenceDepsRouter per-video 路由器

按 video_id 懒加载 InferenceDeps 并缓存，路由 dispatch/prompt_builder:
- create_dispatch: 按 session_id 路由到对应视频的工具调度
- create_prompt_builder: 自动注册 qid→vid 映射并路由 prompt 构建
- 三元组 (video_id, skills_dir, prompts_dir) 缓存键

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/test_deps_router.py

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+"""InferenceDepsRouter 单元测试。"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from app.harness.deps_router import InferenceDepsRouter
+from core.types import GeneratedQuestion
+
+# ---------------------------------------------------------------------------
+# 测试辅助
+# ---------------------------------------------------------------------------
+
+
+def _make_router(
+    store_dir: Path | None = None,
+) -> InferenceDepsRouter:
+    """构造一个 InferenceDepsRouter 实例，所有外部依赖用 MagicMock。"""
+    return InferenceDepsRouter(
+        store_dir=store_dir or Path("/fake/store"),
+        embed_provider=MagicMock(),
+        llm=MagicMock(),
+        vlm=MagicMock(),
+        ocr=None,
+        default_prompts_dir=Path("/fake/prompts"),
+        default_skills_dir=Path("/fake/skills"),
+        skill_mode="none",
+        verify_vision=False,
+        anchor=False,
+        assemble_mode="concat",
+    )
+
+
+def _make_qa(
+    question_id: str = "q1",
+    video_id: str = "vid1",
+) -> GeneratedQuestion:
+    """构造 GeneratedQuestion 测试实例。"""
+    return GeneratedQuestion(
+        question_id=question_id,
+        video_id=video_id,
+        task_type="Action Reasoning",
+        question="test?",
+        options=("A. a", "B. b", "C. c", "D. d"),
+        answer="A",
+        source_nodes=(),
+        difficulty="medium",
+    )
+
+
+def _make_fake_deps() -> MagicMock:
+    """构造 InferenceDeps 替身。"""
+    deps = MagicMock()
+    deps.prompt_builder.return_value = ("system_prompt", "user_prompt")
+    deps.tool_dispatch_fn = AsyncMock(return_value="tool_result")
+    return deps
+
+
+# =========================================================================
+# prompt_builder 测试
+# =========================================================================
+
+
+class TestPromptBuilder:
+    """create_prompt_builder 返回的闭包测试。"""
+
+    def test_registers_qid_to_vid_mapping(self) -> None:
+        """prompt_builder 调用时自动注册 question_id → video_id 映射。"""
+        router = _make_router()
+        fake_deps = _make_fake_deps()
+        router._build_deps = MagicMock(return_value=fake_deps)
+
+        builder = router.create_prompt_builder()
+        qa = _make_qa(question_id="q42", video_id="vid99")
+        builder(qa)
+
+        assert router._qid_to_vid["q42"] == "vid99"
+
+    def test_returns_prompt_from_deps(self) -> None:
+        """prompt_builder 返回 deps.prompt_builder 的结果。"""
+        router = _make_router()
+        fake_deps = _make_fake_deps()
+        router._build_deps = MagicMock(return_value=fake_deps)
+
+        builder = router.create_prompt_builder()
+        qa = _make_qa()
+        result = builder(qa)
+
+        assert result == ("system_prompt", "user_prompt")
+        fake_deps.prompt_builder.assert_called_once_with(qa)
+
+
+# =========================================================================
+# dispatch 测试
+# =========================================================================
+
+
+class TestDispatch:
+    """create_dispatch 返回的闭包测试。"""
+
+    @pytest.mark.asyncio
+    async def test_routes_by_session_id(self) -> None:
+        """dispatch 按 session_id 查找 video_id 并路由到对应 deps。"""
+        router = _make_router()
+        fake_deps = _make_fake_deps()
+        router._build_deps = MagicMock(return_value=fake_deps)
+
+        # 先通过 prompt_builder 注册映射
+        builder = router.create_prompt_builder()
+        qa = _make_qa(question_id="q1", video_id="vid1")
+        builder(qa)
+
+        # 然后 dispatch
+        dispatch = router.create_dispatch()
+        result = await dispatch("search", {"query": "test"}, context={"session_id": "q1"})
+
+        assert result == "tool_result"
+        fake_deps.tool_dispatch_fn.assert_called_once_with(
+            "search", {"query": "test"}, context={"session_id": "q1"}
+        )
+
+    @pytest.mark.asyncio
+    async def test_unknown_session_id_raises_key_error(self) -> None:
+        """未注册的 session_id 抛出 KeyError。"""
+        router = _make_router()
+        dispatch = router.create_dispatch()
+
+        with pytest.raises(KeyError, match="未注册的 session_id"):
+            await dispatch("search", {}, context={"session_id": "unknown"})
+
+    @pytest.mark.asyncio
+    async def test_missing_session_id_raises_key_error(self) -> None:
+        """context 中缺少 session_id 抛出 KeyError。"""
+        router = _make_router()
+        dispatch = router.create_dispatch()
+
+        with pytest.raises(KeyError, match="未注册的 session_id"):
+            await dispatch("search", {}, context={})
+
+
+# =========================================================================
+# 缓存测试
+# =========================================================================
+
+
+class TestCaching:
+    """InferenceDeps 缓存行为测试。"""
+
+    def test_same_video_reuses_cached_deps(self) -> None:
+        """同一 video_id 复用缓存的 InferenceDeps。"""
+        router = _make_router()
+        fake_deps = _make_fake_deps()
+        router._build_deps = MagicMock(return_value=fake_deps)
+
+        builder = router.create_prompt_builder()
+        qa1 = _make_qa(question_id="q1", video_id="vid1")
+        qa2 = _make_qa(question_id="q2", video_id="vid1")
+        builder(qa1)
+        builder(qa2)
+
+        # _build_deps 只调用一次（第二次命中缓存）
+        assert router._build_deps.call_count == 1
+
+    def test_clear_cache_empties_both(self) -> None:
+        """clear_cache 清空 deps 缓存和 qid→vid 映射表。"""
+        router = _make_router()
+        fake_deps = _make_fake_deps()
+        router._build_deps = MagicMock(return_value=fake_deps)
+
+        builder = router.create_prompt_builder()
+        builder(_make_qa())
+
+        assert len(router._deps_cache) == 1
+        assert len(router._qid_to_vid) == 1
+
+        router.clear_cache()
+
+        assert len(router._deps_cache) == 0
+        assert len(router._qid_to_vid) == 0